Your Inner Fish: Finding Ourselves in Ancient Fish PDF

2  CONTENTS Title Page Dedication Preface ONE Finding Your Inner Fish TWO Getting a Grip THREE Handy Genes FOUR Teeth Everywhere FIVE Getting Ahead SIX The Best-Laid (Body) Plans SEVEN Adventures in...

2  CONTENTS Title Page Dedication Preface ONE Finding Your Inner Fish TWO Getting a Grip THREE Handy Genes FOUR Teeth Everywhere FIVE Getting Ahead SIX The Best-Laid (Body) Plans SEVEN Adventures in Bodybuilding EIGHT Making Scents NINE Vision TEN Ears ELEVEN The Meaning of It All Epilogue Notes, References, and Further Reading Acknowledgments Copyright 3 TO MICHELE 4  PREFACE This book grew out of an extraordinary circumstance in my life. On account of faculty departures, I ended up directing the human anatomy course at the medical school of the University of Chicago. Anatomy is the course during which nervous first-year medical students dissect human cadavers while learning the names and organization of most of the organs, holes, nerves, and vessels in the body. This is their grand entrance to the world of medicine, a formative experience on their path to becoming physicians. At first glance, you couldn’t have imagined a worse candidate for the job of training the next generation of doctors: I’m a paleontologist who has spent most of his career working on fish. It turns out that being a paleontologist is a huge advantage in teaching human anatomy. Why? The best road maps to human bodies lie in the bodies of other animals. The simplest way to teach students the nerves in the human head is to show them the state of affairs in sharks. The easiest road map to their limbs lies in fish. Reptiles are a real help with the structure of the brain. The reason is that 5 the bodies of these creatures are often simpler versions of ours. During the summer of my second year leading the course, working in the Arctic, my colleagues and I discovered fossil fish that gave us powerful new insights into the invasion of land by fish over 375 million years ago. That discovery and my foray into teaching human anatomy led me to explore a profound connection. That exploration became this book. 6  CHAPTER ONE FINDING YOUR INNER FISH Typical summers of my adult life are spent in snow and sleet, cracking rocks on cliffs well north of the Arctic Circle. Most of the time I freeze, get blisters, and find absolutely nothing. But if I have any luck, I find ancient fish bones. That may not sound like buried treasure to most people, but to me it is more valuable than gold. Ancient fish bones can be a path to knowledge about who we are and how we got that way. We learn about our own bodies in seemingly bizarre places, ranging from the fossils of worms and fish recovered from rocks from around the world to the DNA in virtually every animal alive on earth today. But that does not explain my confidence about why skeletal remains from the past—and the remains of fish, no less—offer clues about the fundamental structure of our bodies. How can we visualize events that happened millions and, in many cases, billions of years ago? Unfortunately, there were no eyewitnesses; none of us was around. In fact, nothing that talks or has a mouth or even a head was 7 around for most of this time. Even worse, the animals that existed back then have been dead and buried for so long their bodies are only rarely preserved. If you consider that over 99 percent of all species that ever lived are now extinct, that only a very small fraction are preserved as fossils, and that an even smaller fraction still are ever found, then any attempt to see our past seems doomed from the start. DIGGING FOSSILS—SEEING OURSELVES I first saw one of our inner fish on a snowy July afternoon while studying 375-million-year-old rocks on Ellesmere Island, at a latitude about 80 degrees north. My colleagues and I had traveled up to this desolate part of the world to try to discover one of the key stages in the shift from fish to land-living animals. Sticking out of the rocks was the snout of a fish. And not just any fish: a fish with a flat head. Once we saw the flat head we knew we were on to something. If more of this skeleton were found inside the cliff, it would reveal the early stages in the history of our skull, our neck, even our limbs. What did a flat head tell me about the shift from sea to land? More relevant to my personal safety and comfort, why was I in the Arctic and not in Hawaii? The answers to these questions lie in the story of how we find fossils and how we use them to decipher our own past. 8  Fossils are one of the major lines of evidence that we use to understand ourselves. (Genes and embryos are others, which I will discuss later.) Most people do not know that finding fossils is something we can often do with surprising precision and predictability. We work at home to maximize our chances of success in the field. Then we let luck take over. The paradoxical relationship between planning and chance is best described by Dwight D. Eisenhower’s famous remark about warfare: “In preparing for battle, I have found that planning is essential, but plans are useless.” This captures field paleontology in a nutshell. We make all kinds of plans to get us to promising fossil sites. Once we’re there, the entire field plan may be thrown out the window. Facts on the ground can change our best-laid plans. Yet we can design expeditions to answer specific scientific questions. Using a few simple ideas, which I’ll talk about below, we can predict where important fossils might be found. Of course, we are not successful 100 percent of the time, but we strike it rich often enough to make things interesting. I have made a career out of doing just that: finding early mammals to answer questions of mammal origins, the earliest frogs to answer questions of frog origins, and some of the earliest limbed animals to understand the origins of land-living animals. In many ways, field paleontologists have a significantly easier time finding new sites today than we ever did before. We know more about the geology of local areas, thanks to 9 the geological exploration undertaken by local governments and oil and gas companies. The Internet gives us rapid access to maps, survey information, and aerial photos. I can even scan your backyard for promising fossil sites right from my laptop. To top it off, imaging and radiographic devices can see through some kinds of rock and allow us to visualize the bones inside. Despite these advances, the hunt for the important fossils is much what it was a hundred years ago. Paleontologists still need to look at rock—literally to crawl over it—and the fossils within must often be removed by hand. So many decisions need to be made when prospecting for and removing fossil bone that these processes are difficult to automate. Besides, looking at a monitor screen to find fossils would never be nearly as much fun as actually digging for them. What makes this tricky is that fossil sites are rare. To maximize our odds of success, we look for the convergence of three things. We look for places that have rocks of the right age, rocks of the right type to preserve fossils, and rocks that are exposed at the surface. There is another factor: serendipity. That I will show by example. Our example will show us one of the great transitions in the history of life: the invasion of land by fish. For billions of years, all life lived only in water. Then, as of about 365 million years ago, creatures also inhabited land. Life in these two environments is radically different. Breathing in water requires very different organs than breathing in air. 10 The same is true for excretion, feeding, and moving about. A whole new kind of body had to arise. At first glance, the divide between the two environments appears almost unbridgeable. But everything changes when we look at the evidence; what looks impossible actually happened. In seeking rocks of the right age, we have a remarkable fact on our side. The fossils in the rocks of the world are not arranged at random. Where they sit, and what lies inside them, is most definitely ordered, and we can use this order to design our expeditions. Billions of years of change have left layer upon layer of different kinds of rock in the earth. The working assumption, which is easy to test, is that rocks on the top are younger than rocks on the bottom; this is usually true in areas that have a straightforward, layer-cake arrangement (think the Grand Canyon). But movements of the earth’s crust can cause faults that shift the position of the layers, putting older rocks on top of younger ones. Fortunately, once the positions of these faults are recognized, we can often piece the original sequence of layers back together. The fossils inside these rock layers also follow a progression, with lower layers containing species entirely different from those in the layers above. If we could quarry a single column of rock that contained the entire history of life, we would find an extraordinary range of fossils. The lowest layers would contain little visible evidence of life. Layers above them would contain impressions of a diverse set of jellyfish-like things. Layers still higher would have 11 creatures with skeletons, appendages, and various organs, such as eyes. Above those would be layers with the first animals to have backbones. And so on. The layers with the first people would be found higher still. Of course, a single column containing the entirety of earth history does not exist. Rather, the rocks in each location on earth represent only a small sliver of time. To get the whole picture, we need to put the pieces together by comparing the rocks themselves and the fossils inside them, much as if working a giant jigsaw puzzle. That a column of rocks has a progression of fossil species probably comes as no surprise. Less obvious is that we can make detailed predictions about what the species in each layer might actually look like by comparing them with species of animals that are alive today; this information helps us to predict the kinds of fossils we will find in ancient rock layers. In fact, the fossil sequences in the world’s rocks can be predicted by comparing ourselves with the animals at our local zoo or aquarium. How can a walk through the zoo help us predict where we should look in the rocks to find important fossils? A zoo offers a great variety of creatures that are all distinct in many ways. But let’s not focus on what makes them distinct; to pull off our prediction, we need to focus on what different creatures share. We can then use the features common to all species to identify groups of creatures with similar traits. All the living things can be organized and arranged like a set of Russian nesting dolls, with smaller 12 groups of animals comprised in bigger groups of animals. When we do this, we discover something very fundamental about nature. Every species in the zoo and the aquarium has a head and two eyes. Call these species “Everythings.” A subset of the creatures with a head and two eyes has limbs. Call the limbed species “Everythings with limbs.” A subset of these headed and limbed creatures has a huge brain, walks on two feet, and speaks. That subset is us, humans. We could, of course, use this way of categorizing things to make many more subsets, but even this threefold division has predictive power. The fossils inside the rocks of the world generally follow this order, and we can put it to use in designing new expeditions. To use the example above, the first member of the group “Everythings,” a creature with a head and two eyes, is found in the fossil record well before the first “Everything with limbs.” More precisely, the first fish (a card-carrying member of the “Everythings”) appears before the first amphibian (an “Everything with limbs”). Obviously, we refine this by looking at more kinds of animals and many more characteristics that groups of them share, as well as by assessing the actual age of the rocks themselves. In our labs, we do exactly this type of analysis with thousands upon thousands of characteristics and species. We look at every bit of anatomy we can, and often at large chunks of DNA. There is so much data that we often need 13 powerful computers to show us the groups within groups. This approach is the foundation of biology, because it enables us to make hypotheses about how creatures are related to one another. Besides helping us refine the groupings of life, hundreds of years of fossil collection have produced a vast library, or catalogue, of the ages of the earth and the life on it. We can now identify general time periods when major changes occurred. Interested in the origin of mammals? Go to rocks from the period called the Early Mesozoic; geochemistry tells us that these rocks are likely about 210 million years old. Interested in the origin of primates? Go higher in the rock column, to the Cretaceous period, where rocks are about 80 million years old. The order of fossils in the world’s rocks is powerful evidence of our connections to the rest of life. If, digging in 600-million-year-old rocks, we found the earliest jellyfish lying next to the skeleton of a woodchuck, then we would have to rewrite our texts. That woodchuck would have appeared earlier in the fossil record than the first mammal, reptile, or even fish—before even the first worm. Moreover, our ancient woodchuck would tell us that much of what we think we know about the history of the earth and life on it is wrong. Despite more than 150 years of people looking for fossils—on every continent of earth and in virtually every rock layer that is accessible—this observation has never been made. 14  What we discover on our walk through the zoo mirrors how fossils are laid out in the rocks of the world. Let’s now return to our problem of how to find relatives of the first fish to walk on land. In our grouping scheme, these creatures are somewhere between the “Everythings” and the “Everythings with limbs.” Map this to what we know of the rocks, and there is strong geological evidence that the period from 380 million to 365 million years ago is 15 the critical time. The younger rocks in that range, those about 360 million years old, include diverse kinds of fossilized animals that we would all recognize as amphibians or reptiles. My colleague Jenny Clack at Cambridge University and others have uncovered amphibians from rocks in Greenland that are about 365 million years old. With their necks, their ears, and their four legs, they do not look like fish. But in rocks that are about 385 million years old, we find whole fish that look like, well, fish. They have fins, conical heads, and scales; and they have no necks. Given this, it is probably no great surprise that we should focus on rocks about 375 million years old to find evidence of the transition between fish and land-living animals. We have settled on a time period to research, and so have identified the layers of the geological column we wish to investigate. Now the challenge is to find rocks that were formed under conditions capable of preserving fossils. Rocks form in different kinds of environments and these initial settings leave distinct signatures on the rock layers. Volcanic rocks are mostly out. No fish that we know of can live in lava. And even if such a fish existed, its fossilized bones would not survive the superheated conditions in which basalts, rhyolites, granites, and other igneous rocks are formed. We can also ignore metamorphic rocks, such as schist and marble, for they have undergone either superheating or extreme pressure since their initial formation. Whatever fossils might have been preserved in 16 them have long since disappeared. Ideal to preserve fossils are sedimentary rocks: limestones, sandstones, silt-stones, and shales. Compared with volcanic and metamorphic rocks, these are formed by more gentle processes, including the action of rivers, lakes, and seas. Not only are animals likely to live in such environments, but the sedimentary processes make these rocks more likely places to preserve fossils. For example, in an ocean or lake, particles constantly settle out of the water and are deposited on the bottom. Over time, as these particles accumulate, they are compressed by new, overriding layers. The gradual compression, coupled with chemical processes happening inside the rocks over long periods of time, means that any skeletons contained in the rocks stand a decent chance of fossilizing. Similar processes happen in and along streams. The general rule is that the gentler the flow of the stream or river, the better preserved the fossils. Every rock sitting on the ground has a story to tell: the story of what the world looked like as that particular rock formed. Inside the rock is evidence of past climates and surroundings often vastly different from those of today. Sometimes, the disconnect between present and past could not be sharper. Take the extreme example of Mount Everest, near whose top, at an altitude of over five miles, lie rocks from an ancient sea floor. Go to the North Face almost within sight of the famous Hillary Step, and you can find fossilized seashells. Similarly, where we work in the Arctic, temperatures can reach minus 40 degrees Fahrenheit in the 17 winter. Yet inside some of the region’s rocks are remnants of an ancient tropical delta, almost like the Amazon: fossilized plants and fish that could have thrived only in warm, humid locales. The presence of warm-adapted species at what today are extreme altitudes and latitudes attests to how much our planet can change: mountains rise and fall, climates warm and cool, and continents move about. Once we come to grips with the vastness of time and the extraordinary ways our planet has changed, we will be in a position to put this information to use in designing new fossil-hunting expeditions. If we are interested in understanding the origin of limbed animals, we can now restrict our search to rocks that are roughly 375 million to 380 million years old and that were formed in oceans, lakes, or streams. Rule out volcanic rocks and metamorphic rocks, and our search image for promising sites comes into better focus. We are only partly on the way to designing a new expedition, however. It does us no good if our promising sedimentary rocks of the right age are buried deep inside the earth, or if they are covered with grass, or shopping malls, or cities. We’d be digging blindly. As you can imagine, drilling a well hole to find a fossil offers a low probability of success, rather like throwing darts at a dartboard hidden behind a closet door. The best places to look are those where we can walk for miles over the rock to discover areas where bones are “weathering out.” Fossil bones are often harder than the 18 surrounding rock and so erode at a slightly slower rate and present a raised profile on the rock surface. Consequently, we like to walk over bare bedrock, find a smattering of bones on the surface, then dig in. So here is the trick to designing a new fossil expedition: find rocks that are of the right age, of the right type (sedimentary), and well exposed, and we are in business. Ideal fossil-hunting sites have little soil cover and little vegetation, and have been subject to few human disturbances. Is it any surprise that a significant fraction of discoveries happen in desert areas? In the Gobi Desert. In the Sahara. In Utah. In Arctic deserts, such as Greenland. This all sounds very logical, but let’s not forget serendipity. In fact, it was serendipity that put our team onto the trail of our inner fish. Our first important discoveries didn’t happen in a desert, but along a roadside in central Pennsylvania where the exposures could hardly have been worse. To top it off, we were looking there only because we did not have much money. It takes a lot of money and time to go to Greenland or the Sahara Desert. In contrast, a local project doesn’t require big research grants, only money for gas and turnpike tolls. These are critical variables for a young graduate student or a newly hired college teacher. When I started my first job in Philadelphia, the lure was a group of rocks collectively known as the Catskill Formation of Pennsylvania. This formation has been extensively studied for over 150 years. Its age was well known and spanned the Late Devonian. In 19 addition, its rocks were perfect to preserve early limbed animals and their closest relatives. To understand this, it is best to have an image of what Pennsylvania looked like back in the Devonian. Remove the image of present-day Philadelphia, Pittsburgh, or Harrisburg from your mind and think of the Amazon River delta. There were highlands in the eastern part of the state. A series of streams running east to west drained these mountains, ending in a large sea where Pittsburgh is today. It is hard to imagine better conditions to find fossils, except that central Pennsylvania is covered in towns, forests, and fields. As for the exposures, they are mostly where the Pennsylvania Department of Transportation (PennDOT) has decided to put big roads. When PennDOT builds a highway, it blasts. When it blasts, it exposes rock. It’s not always the best exposure, but we take what we can get. With cheap science, you get what you pay for. And then there is also serendipity of a different order: in 1993, Ted Daeschler arrived to study paleontology under my supervision. This partnership was to change both our lives. Our different temperaments are perfectly matched: I have ants in my pants and am always thinking of the next place to look; Ted is patient and knows when to sit on a site to mine it for its riches. Ted and I began a survey of the Devonian rocks of Pennsylvania in hopes of finding new evidence on the origin of limbs. We began by driving to virtually every large roadcut in the eastern part of the state. To our great surprise, shortly after we began the survey, 20 Ted found a marvelous shoulder bone. We named its owner Hynerpeton, a name that translates from Greek as “little creeping animal from Hyner.” Hyner, Pennsylvania, is the nearest town. Hynerpeton had a very robust shoulder, which indicates a creature that likely had very powerful appendages. Unfortunately, we were never able to find the whole skeleton of the animal. The exposures were too limited. By? You guessed it: vegetation, houses, and shopping malls. Along the roads in Pennsylvania, we were looking at an ancient river delta, much like the Amazon today. The state of Pennsylvania (bottom) with the Devonian topography mapped above it. 21  After the discovery of Hynerpeton and other fossils from these rocks, Ted and I were champing at the bit for better- exposed rock. If our entire scientific enterprise was going to be based on recovering bits and pieces, then we could address only very limited questions. So we took a “textbook” approach, looking for well-exposed rocks of the right age and the right type in desert regions, meaning that we wouldn’t have made the biggest discovery of our careers if not for an introductory geology textbook. Originally we were looking at Alaska and the Yukon as potential venues for a new expedition, largely because of relevant discoveries made by other teams. We ended up getting into a bit of an argument/debate about some geological esoterica, and in the heat of the moment, one of us pulled the lucky geology textbook from a desk. While riffling through the pages to find out which one of us was right, we found a diagram. The diagram took our breath away; it showed everything we were looking for. The argument stopped, and planning for a new field expedition began. On the basis of previous discoveries made in slightly younger rocks, we believed that ancient freshwater streams were the best environment in which to begin our hunt. This diagram showed three areas with Devonian freshwater rocks, each with a river delta system. First, there is the east coast of Greenland. This is home to Jenny Clack’s fossil, a very early creature with limbs and one of the earliest known tetrapods. Then there is eastern North America, 22 where we had already worked, home to Hynerpeton. And there is a third area, large and running east–west across the Canadian Arctic. There are no trees, dirt, or cities in the Arctic. The chances were good that rocks of the right age and type would be extremely well exposed. The Canadian Arctic exposures were well known, particularly to the Canadian geologists and paleobotanists who had already mapped them. In fact, Ashton Embry, the leader of the teams that did much of this work, had described the geology of the Devonian Canadian rocks as identical in many ways to the geology of Pennsylvania’s. Ted and I were ready to pack our bags the minute we read this phrase. The lessons we had learned on the highways of Pennsylvania could help us in the High Arctic of Canada. Remarkably, the Arctic rocks are even older than the fossil beds of Greenland and Pennsylvania. So the area perfectly fit all three of our criteria: age, type, and exposure. Even better, it was unknown to vertebrate paleontologists, and therefore un-prospected for fossils. 23  The map that started it all. This map of North America captures what we look for in a nutshell. The different kinds of shading reflect where Devonian age rocks, whether marine or freshwater, are exposed. Three areas that were once river deltas are labeled. Modified from figure 13.1, R. H. Dott and R. L. Batten, Evolution of the Earth (New York: McGraw-Hill, 1988). Reproduced with the permission of The McGraw-Hill Companies. Our new challenges were totally different from those we 24 faced in Pennsylvania. Along the highways in Pennsylvania, we risked being hit by the trucks that whizzed by as we looked for fossils. In the Arctic we risked being eaten by polar bears, running out of food, or being marooned by bad weather. No longer could we pack sandwiches in the car and drive to the fossil beds. We now had to spend at least eight days planning for every single day spent in the field, because the rocks were accessible only by air and the nearest supply base was 250 miles away. We could fly in only enough food and supplies for our crew, plus a slender safety margin. And, most important, the plane’s strict weight limits meant that we could take out only a small fraction of the fossils that we found. Couple those limitations with the short window of time during which we can actually work in the Arctic every year, and you can see that the frustrations we faced were completely new and daunting. Enter my graduate adviser, Dr. Farish A. Jenkins, Jr., from Harvard. Farish had led expeditions to Greenland for years and had the experience necessary to pull this venture off. The team was set. Three academic generations: Ted, my former student; Farish, my graduate adviser; and I were going to march up to the Arctic to try to discover evidence of the shift from fish to land-living animal. There is no field manual for Arctic paleontology. We received gear recommendations from friends and colleagues, and we read books—only to realize that nothing could prepare us for the experience itself. At no time is this 25 more sharply felt than when the helicopter drops one off for the first time in some godforsaken part of the Arctic totally alone. The first thought is of polar bears. I can’t tell you how many times I’ve scanned the landscape looking for white specks that move. This anxiety can make you see things. In our first week in the Arctic, one of the crew saw a moving white speck. It looked like a polar bear about a quarter mile away. We scrambled like Keystone Kops for our guns, flares, and whistles until we discovered that our bear was a white Arctic hare two hundred feet away. With no trees or houses by which to judge distance, you lose perspective in the Arctic. The Arctic is a big, empty place. The rocks we were interested in are exposed over an area about 1,500 kilometers wide. The creatures we were looking for were about four feet long. Somehow, we needed to home in on a small patch of rock that had preserved our fossils. Reviewers of grant proposals can be a ferocious lot; they light on this kind of difficulty all the time. A reviewer for one of Farish’s early Arctic grant proposals put it best. As this referee wrote in his review of the proposal (not cordially, I might add), the odds of finding new fossils in the Arctic were “worse than finding the proverbial needle in the haystack.” It took us four expeditions to Ellesmere Island over six years to find our needle. So much for serendipity. We found what we were looking for by trying, failing, and learning from our failures. Our first sites, in the 1999 field 26 season, were way out in the western part of the Arctic, on Melville Island. We did not know it, but we had been dropped off on the edge of an ancient ocean. The rocks were loaded with fossils, and we found many different kinds of fish. The problem was that they all seemed to be deep- water creatures, not the kind we would expect to find in the shallow streams or lakes that gave rise to land-living animals. Using Ashton Embry’s geological analysis, in 2000 we decided to move the expedition east to Ellesmere Island, because there the rocks would contain ancient streambeds. It did not take long for us to begin finding pieces of fish bones about the size of a quarter preserved as fossils. Our camp (top) looks tiny in the vastness of the landscape. My summer home (bottom) is a small tent, 27  usually surrounded by piles of rocks to protect it from fifty-mile-per-hour winds. Photographs by the author. The real breakthrough came toward the end of the field season in 2000. It was just before dinner, about a week before our scheduled pickup to return home. The crew had come back to camp, and we were involved in our early- evening activities: organizing the day’s collections, preparing field notes, and beginning to assemble dinner. Jason Downs, then a college undergraduate eager to learn paleontology, hadn’t returned to camp on time. This is a cause for worry, as we typically go out in teams; or if we separate, we give each other a definite schedule of when we will make contact again. With polar bears in the area and fierce storms that can roll in unexpectedly, we do not take any chances. I remember sitting in the main tent with the crew, the worry about Jason building with each passing moment. As we began to concoct a search plan, I heard the zipper on the tent open. At first all I saw was Jason’s head. He had a wild-eyed expression on his face and was out of breath. As Jason entered the tent, we knew we were not dealing with a polar bear emergency; his shotgun was still shouldered. The cause of his delay became clear as his still shaking hand pulled out handful after handful of fossil bones that had been stuffed into every pocket: his coat, pants, inner shirt, and daypack. I imagine he would have stuffed his socks and shoes if he could have walked home that way. All of these little fossil bones were on the surface of a small site, no bigger than a parking spot for a compact 28 car, about a mile away from camp. Dinner could wait. With twenty-four hours of daylight in the Arctic summer, we did not have to worry about the setting sun, so we grabbed chocolate bars and set off for Jason’s site. It was on the side of a hill between two beautiful river valleys and, as Jason had discovered, was covered in a carpet of fossil fish bones. We spent a few hours picking up the fragments, taking photos, and making plans. This site had all the makings of precisely what we were looking for. We returned the next day with a new goal: to find the exact layer of rock that contained the bones. The trick was to identify the source of Jason’s mess of bone fragments—our only hope of finding intact skeletons. The problem was the Arctic environment. Each winter, the temperature sinks to minus 40 degrees Fahrenheit. In the summer, when the sun never sets, the temperature rises to nearly 50 degrees. The resulting freeze-thaw cycle crumbles the surface rocks and fossils. Each winter they cool and shrink; each summer they heat and expand. As they shrink and swell with each season over thousands of years at the surface, the bones fall apart. Confronted by a jumbled mass of bone spread across the hill, we could not identify any obvious rock layer as their source. We spent several days following the fragment trails, digging test pits, practically using our geological hammers as divining rods to see where in the cliff the bones were emerging. After four days, we exposed the layer and eventually found skeleton upon skeleton of fossil fish, often lying one on top of another. We 29 spent parts of two summers exposing these fish. This is where we work: southern Ellesmere Island, in Nunavut Territory, Canada, 1,000 miles from the North Pole. Failure again: all the fish we were finding were well- known species that had been collected in sites of a similar age in Eastern Europe. To top it off, these fish weren’t very closely related to land-living animals. In 2004, we decided to give it one more try. This was a do-or-die situation. The Arctic expeditions were prohibitively expensive and, short 30 of a remarkable discovery, we would have to call it quits. Everything changed over a period of four days in early July 2004. I was flipping rock at the bottom of the quarry, cracking ice more often than rock. I cracked the ice and saw something that I will never forget: a patch of scales unlike anything else we had yet seen in the quarry. This patch led to another blob covered by ice. It looked like a set of jaws. They were, however, unlike the jaws of any fish I had ever seen. They looked as if they might have connected to a flat head. One day later, my colleague Steve Gatesy was flipping rocks at the top of the quarry. Steve removed a fist-size rock to reveal the snout of an animal looking right out at him. Like my ice-covered fish at the bottom of the pit, it had a flat head. It was new and important. But unlike my fish, Steve’s had real potential. We were looking at the front end, and with luck the rest of the skeleton might be safely sitting in the cliff. Steve spent the rest of the summer removing rock from it bit by bit so that we could bring the entire skeleton back to the lab and clean it up. Steve’s masterful work with this specimen led to the recovery of one of the finest fossils discovered to date at the water–land transition. The specimens we brought back to the lab at home were little more than boulders with fossils inside. Over the course of two months, the rock was removed piece by piece, often manually with dental tools or small picks by the preparators in the lab. Every day a new piece of the fossil 31 creature’s anatomy was revealed. Almost every time a large section was exposed, we learned something new about the origin of land-living animals. What we saw gradually emerge from these rocks during the fall of 2004 was a beautiful intermediate between fish and land-living animals. Fish and land-living animals differ in many respects. Fish have conical heads, whereas the earliest land-living animals have almost crocodile-like heads—flat, with the eyes on top. Fish do not have necks: their shoulders are attached to their heads by a series of bony plates. Early land-living animals, like all their descendants, do have necks, meaning their heads can bend independently of their shoulders. There are other big differences. Fish have scales all over their bodies; land-living animals do not. Also, importantly, fish have fins, whereas land-living animals have limbs with fingers, toes, wrists, and ankles. We can continue these comparisons and make a very long list of the ways that fish differ from land-living animals. 32  The process of finding fossils begins with a mass in a rock that is gradually removed over time. Here I show a fossil as it travels from the field to the lab and is carefully prepared as a specimen: the skeleton of the new animal. Photograph in upper left by author; other photographs courtesy of Ted Daeschler, Academy of Natural Sciences of Philadelphia. But our new creature broke down the distinction between these two different kinds of animal. Like a fish, it has scales on its back and fins with fin webbing. But, like early land-living animals, it has a flat head and a neck. And, when we look inside the fin, we see bones that correspond to the upper arm, the forearm, even parts of the wrist. The joints are there, too: this is a fish with shoulder, elbow, and wrist joints. All inside a fin with webbing. Virtually all of the features that this creature shares with land-living creatures look very primitive. For example, the 33 shape and various ridges on the fish’s upper “arm” bone, the humerus, look part fish and part amphibian. The same is true of the shape of the skull and the shoulder. It took us six years to find it, but this fossil confirmed a prediction of paleontology: not only was the new fish an intermediate between two different kinds of animal, but we had found it also in the right time period in earth’s history and in the right ancient environment. The answer came from 375-million-year-old rocks, formed in ancient streams. This figure says it all. Tiktaalik is intermediate between fish and primitive land-living animal. As the discoverers of the creature, Ted, Farish, and I had the privilege of giving it a formal scientific name. We wanted the name to reflect the fish’s provenance in the Nunavut Territory of the Arctic and the debt we owed to the 34 Inuit people for permission to work there. We engaged the Nunavut Council of Elders, formally known as the Inuit Qaujimajatuqangit Katimajiit, to come up with a name in the Inuktitut language. My obvious concern was that a committee named Inuit Qaujimajatuqangit Katimajiit might not propose a scientific name we could pronounce. I sent them a picture of the fossil, and the elders came up with two suggestions, Siksagiaq and Tiktaalik. We went with Tiktaalik for its relative ease of pronunciation for the non- Inuktitut-speaking tongue and because of its meaning in Inuktitut: “large freshwater fish.” Tiktaalik was the lead story in a number of newspapers the day after the find was announced in April 2006, including above-the-fold headlines in such places as The New York Times. This attention ushered in a week unlike any other in my normally quiet life. Though for me the greatest moment of the whole media blitz was not seeing the political cartoons or reading the editorial coverage and the heated discussions on the blogs. It took place at my son’s preschool. In the midst of the press hubbub, my son’s preschool teacher asked me to bring in the fossil and describe it. I dutifully brought a cast of Tiktaalik into Nathaniel’s class, bracing myself for the chaos that would ensue. The twenty four-and five-year-olds were surprisingly well behaved as I described how we had worked in the Arctic to find the fossil and showed them the animal’s sharp teeth. Then I asked what they thought it was. Hands shot up. The first child said 35 it was a crocodile or an alligator. When queried why, he said that like a crocodile or lizard it has a flat head with eyes on top. Big teeth, too. Other children started to voice their dissent. Choosing the raised hand of one of these kids, I heard: No, no, it isn’t a crocodile, it is a fish, because it has scales and fins. Yet another child shouted, “Maybe it is both.” Tiktaalik’s message is so straightforward even preschoolers can see it. For our purposes, there is an even more profound take on Tiktaalik. This fish doesn’t just tell us about fish; it also contains a piece of us. The search for this connection is what led me to the Arctic in the first place. How can I be so sure that this fossil says something about my own body? Consider the neck of Tiktaalik. All fish prior to Tiktaalik have a set of bones that attach the skull to the shoulder, so that every time the animal bent its body, it also bent its head. Tiktaalik is different. The head is completely free of the shoulder. This whole arrangement is shared with amphibians, reptiles, birds, and mammals, including us. The entire shift can be traced to the loss of a few small bones in a fish like Tiktaalik. 36  Tracing arm bones from fish to humans. I can do a similar analysis for the wrists, ribs, ears, and other parts of our skeleton—all these features can be traced back to a fish like this. This fossil is just as much a part of our history as the African hominids, such as Australopithecus afarensis, the famous “Lucy.” Seeing Lucy, we can understand our history as highly advanced primates. Seeing Tiktaalik is seeing our history as fish. So what have we learned? Our world is so highly ordered that we can use a walk through a zoo to predict the kinds of fossils that lie in the different layers of rocks around the world. Those predictions can bring about fossil discoveries that tell us about ancient events in the history of life. The record of those events remains inside us, as part of our 37 anatomical organization. What I haven’t mentioned is that we can also trace our history inside our genes, through DNA. This record of our past doesn’t lie in the rocks of the world; it lies in every cell inside us. We’ll use both fossils and genes to tell our story, the story of the making of our bodies. 38  CHAPTER TWO GETTING A GRIP Images of the medical school anatomy lab are impossible to forget. Imagine walking into a room where you will spend several months taking a human body apart layer by layer, organ by organ, all as a way to learn tens of thousands of new names and body structures. In the months before I did my first human dissection, I readied myself by trying to envision what I would see, how I would react, and what I would feel. It turned out that my imagined world in no way prepared me for the experience. The moment when we removed the sheet and saw the body for the first time wasn’t nearly as stressful as I’d expected. We were to dissect the chest, so we exposed it while leaving the head, arms, and legs wrapped in preservative-drenched gauze. The tissues did not look very human. Having been treated with a number of preservatives, the body didn’t bleed when cut, and the skin and internal organs had the consistency of rubber. I began to think that the cadaver looked more like a doll than a human. A few weeks went by as we exposed the organs of the chest and abdomen. I came 39 to think that I was quite the pro; having already seen most of the internal organs, I had developed a cocky self- confidence about the whole experience. I did my initial dissections, made my cuts, and learned the anatomy of most of the major organs. It was all very mechanical, detached, and scientific. This comfortable illusion was rudely shattered when I uncovered the hand. As I unwrapped the gauze from the fingers—as I saw the joints, fingertips, and fingernails for the first time—I uncovered emotions that had been concealed during the previous few weeks. This was no doll or mannequin; this had once been a living person, who used that hand to carry and caress. Suddenly, this mechanical exercise, dissection, became deeply and emotionally personal. Until that moment, I was blind to my connection to the cadaver. I had already exposed the stomach, the gallbladder, and other organs; but what sane person forms a human connection at the sight of a gallbladder? What is it about a hand that seems quintessentially human? The answer must, at some level, be that the hand is a visible connection between us; it is a signature for who we are and what we can attain. Our ability to grasp, to build, and to make our thoughts real lies inside this complex of bones, nerves, and vessels. The immediate thing that strikes you when you see the inside of the hand is its compactness. The ball of your thumb, the thenar eminence, contains four different muscles. Twiddle your thumb and tilt your hand: ten 40 different muscles and at least six different bones work in unison. Inside the wrist are at least eight small bones that move against one another. Bend your wrist, and you are using a number of muscles that begin in your forearm, extending into tendons as they travel down your arm to end at your hand. Even the simplest motion involves a complex interplay among many parts packed in a small space. The relationship between complexity and humanity within our hands has long fascinated scientists. In 1822, the eminent Scottish surgeon Sir Charles Bell wrote the classic book on the anatomy of hands. The title says it all: The Hand, Its Mechanism and Vital Endowments as Evincing Design. To Bell, the structure of the hand was “perfect” because it was complex and ideally arranged for the way we live. In his eye, this designed perfection could only have a divine origin. The great anatomist Sir Richard Owen was one of the scientific leaders in this search for divine order within bodies. He was fortunate to be an anatomist in the mid- 1800s, when there were still entirely new kinds of animals to discover living in the distant reaches of the earth. As more and more parts of the world were explored by westerners, all sorts of exotic creatures made their way back to laboratories and museums. Owen described the first gorilla, brought back from expeditions to central Africa. He coined the name “dinosaur” for a new kind of fossil creature discovered in rocks in England. His study of these bizarre new creatures gave him special insights: he 41 began to see important patterns in the seeming chaos of life’s diversity. Owen discovered that our arms and legs, our hands and feet, fit into a larger scheme. He saw what anatomists before him had long known, that there is a pattern to the skeleton of a human arm: one bone in the upper arm, two bones in the forearm, a bunch of nine little bones at the wrists, then a series of five rods that make the fingers. The pattern of bones in the human leg is much the same: one bone, two bones, lotsa blobs, and five toes. In comparing this pattern with the diversity of skeletons in the world, Owen made a remarkable discovery. Owen’s genius was not that he focused on what made the various skeletons different. What he found, and later promoted in a series of lectures and volumes, were exceptional similarities among creatures as different as frogs and people. All creatures with limbs, whether those limbs are wings, flippers, or hands, have a common design. One bone, the humerus in the arm or the femur in the leg, articulates with two bones, which attach to a series of small blobs, which connect with the fingers or toes. This pattern underlies the architecture of all limbs. Want to make a bat wing? Make the fingers really long. Make a horse? Elongate the middle fingers and toes and reduce and lose the outer ones. How about a frog leg? Elongate the bones of the leg and fuse several of them together. The differences between creatures lie in differences in the shapes and sizes of the bones and the numbers of blobs, fingers, and toes. Despite 42 radical changes in what limbs do and what they look like, this underlying blueprint is always present. The common plan for all limbs: one bone, followed by two bones, then little blobs, then fingers or toes. For Owen, seeing a design in the limbs was only the beginning: when he looked at skulls and backbones, indeed when he considered the entire architecture of the body, he 43 found the same thing. There is a fundamental design in the skeleton of all animals. Frogs, bats, humans, and lizards are all just variations on a theme. That theme, to Owen, was the plan of the Creator. Shortly after Owen announced this observation in his classic monograph On the Nature of Limbs, Charles Darwin supplied an elegant explanation for it. The reason the wing of a bat and the arm of a human share a common skeletal pattern is because they shared a common ancestor. The same reasoning applies to human arms and bird wings, human legs and frog legs—everything that has limbs. There is a major difference between Owen’s theory and that of Darwin: Darwin’s theory allows us to make very precise predictions. Following Darwin, we would expect to find that Owen’s blueprint has a history that will be revealed in creatures with no limbs at all. Where, then, do we look for the history of the limb pattern? We look to fish and their fin skeletons. SEEING THE FISH In Owen and Darwin’s day, the gulf between fins and limbs seemed impossibly wide. Fish fins don’t have any obvious similarities to limbs. On the outside, most fish fins are largely made up of fin webbing. Our limbs have nothing like this, nor do the limbs of any other creature alive today. The comparisons do not get any easier when you remove the fin 44 webbing to see the skeleton inside. In most fish, there is nothing that can be compared to Owen’s one bone–two bones–lotsa blobs–digits pattern. All limbs have a single long bone at their base: the humerus in the upper arm and the femur in the upper leg. In fish, the whole skeleton looks utterly different. The base of a typical fin has four or more bones inside. In the mid-1800s, anatomists began to learn of mysterious living fish from the southern continents. One of the first was discovered by German anatomists working in South America. It looked like a normal fish, with fins and scales, but behind its throat were large vascular sacs: lungs. Yet the creature had scales and fins. So confused were the discoverers that they named the creature Lepidosiren paradoxa, “paradoxically scaled amphibian.” Other fish with lungs, aptly named lungfish, were soon found in Africa and Australia. African explorers brought one to Owen. Scientists such as Thomas Huxley and the anatomist Carl Gegenbaur found lungfish to be essentially a cross between an amphibian and a fish. Locals found them delicious. A seemingly trivial pattern in the fins of these fish had a profound impact on science. The fins of lungfish have at their base a single bone that attaches to the shoulder. To anatomists, the comparison was obvious. Our upper arm has a single bone, and that single bone, the humerus, attaches to our shoulder. In the lungfish, we have a fish with a humerus. And, curiously, it is not just any fish; it is a fish that also has lungs. Coincidence? 45  As a handful of these living species became known in the 1800s, clues started to come from another source. As you might guess, these insights came from ancient fish. One of the first of these fossils came from the shores of the Gaspé Peninsula in Quebec, in rocks about 380 million years old. The fish was given a tongue-twister name, Eusthenopteron. Eusthenopteron had a surprising mix of features seen in amphibians and fish. Of Owen’s one bone– two bones–lotsa blobs–digits plan of limbs, Eusthenopteron had the one bone–two bones part, but in a fin. Some fish, then, had structures like those in a limb. Owen’s archetype was not a divine and eternal part of all life. It had a history, and that history was to be found in Devonian age rocks, rocks that are between 390 million and 360 million years old. This profound insight defined a whole new research program with a whole new research agenda: somewhere in the Devonian rocks we should find the origin of fingers and toes. In the 1920s, the rocks provided more surprises. A young Swedish paleontologist, Gunnar Save-Soderbergh, was given the extraordinary opportunity to explore the east coast of Greenland for fossils. The region was terra incognita, but Save-Soderbergh recognized that it featured enormous deposits of Devonian rocks. He was one of the exceptional field paleontologists of all time, who throughout his short career uncovered remarkable fossils with both a bold exploring spirit and a precise attention to detail. (Unfortunately, he was to die tragically of 46 tuberculosis at a young age, soon after the stunning success of his field expeditions.) In expeditions between 1929 and 1934, Save-Soderbergh’s team discovered what, at the time, was labeled a major missing link. Newspapers around the world trumpeted his discovery; editorials analyzed its importance; cartoons lampooned it. The fossils in question were true mosaics: they had fish-like heads and tails, yet they also had fully formed limbs (with fingers and toes), and vertebrae that were extraordinarily amphibian-like. After Save-Soderbergh died, the fossils were described by his colleague Erik Jarvik, who named one of the new species Ichthyostega soderberghi in honor of his friend. 47 The fins of most fish—for example, a zebrafish (top)— have large amounts of fin webbing and many bones at the base. Lungfish captured people’s interest because like us they have a single bone at the base of the appendage. Eusthenopteron (middle) showed how fossils begin to fill the gap; it has bones that compare to our upper arm and forearm. Acanthostega (bottom) shares Eusthenopteron’s pattern of arm bones with the addition of fully formed digits. 48  For our story, Ichthyostega is a bit of a letdown. True, it is a remarkable intermediate in most aspects of its head and back, but it says very little about the origin of limbs because, like any amphibian, it already has fingers and toes. Another creature, one that received little notice when Save- Soderbergh announced it, was to provide real insights decades later. This second limbed animal was to remain an enigma until 1988, when a paleontological colleague of mine, Jenny Clack, who we introduced in the first chapter, returned to Save-Soderbergh’s sites and found more of its fossils. The creature, called Acanthostega gunnari back in the 1920s on the basis of Save-Soderbergh’s fragments, now revealed full limbs, with fingers and toes. But it also carried a real surprise: Jenny found that the limb was shaped like a flipper, almost like that of a seal. This suggested to her that the earliest limbs arose to help animals swim, not walk. That insight was a significant advance, but a problem remained: Acanthostega had fully formed digits, with a real wrist and no fin webbing. Acanthostega had a limb, albeit a very primitive one. The search for the origins of hands and feet, wrists and ankles had to go still deeper in time. This is where matters stood until 1995. FINDING FISH FINGERS AND WRISTS In 1995, Ted Daeschler and I had just returned to his house 49 in Philadelphia after driving all through central Pennsylvania in an effort to find new roadcuts. We had found a lovely cut on Route 15 north of Williamsport, where PennDOT had created a giant cliff in sandstones about 365 million years old. The agency had dynamited the cliff and left piles of boulders alongside the highway. This was perfect fossil-hunting ground for us, and we stopped to crawl over the boulders, many of them roughly the size of a small microwave oven. Some had fish scales scattered throughout, so we decided to bring a few back home to Philadelphia. Upon our return to Ted’s house, his four-year- old daughter, Daisy, came running out to see her dad and asked what we had found. In showing Daisy one of the boulders, we suddenly realized that sticking out of it was a sliver of fin belonging to a large fish. We had completely missed it in the field. And, as we were to learn, this was no ordinary fish fin: it clearly had lots of bones inside. People in the lab spent about a month removing the fin from the boulder—and there, exposed for the first time, was a fish with Owen’s pattern. Closest to the body was one bone. This one bone attached to two bones. Extending away from the fin were about eight rods. This looked for all the world like a fish with fingers. Our fin had a full set of webbing, scales, and even a fish- like shoulder, but deep inside were bones that corresponded to much of the “standard” limb. Unfortunately, we had only an isolated fin. What we needed was to find a place where whole bodies of creatures could 50 be recovered intact. A single isolated fin could never help us answer the real questions: What did the creature use its fins for, and did the fish fins have bones and joints that worked like ours? The answer would come only from whole skeletons. Our tantalizing fin. Sadly, we found only this isolated specimen. Stipple diagram used with the permission of Scott Rawlins, Arcadia University. Photo by the author. 51  For that find, we had to search almost ten years. And I wasn’t the first to recognize what we were looking at. The first were two professional fossil preparators, Fred Mullison and Bob Masek. Preparators use dental tools to scratch at the rocks we find in the field and thereby expose the fossils inside. It can take months, if not years, for a preparator to turn a big fossil-filled boulder like ours into a beautiful, research-quality specimen. During the 2004 expedition, we had collected three chunks of rock, each about the size of a piece of carry-on luggage, from the Devonian of Ellesmere Island. Each contained a flat-headed animal: the one I found in ice at the bottom of the quarry, Steve’s specimen, and a third specimen we discovered in the final week of the expedition. In the field we had removed each head, leaving enough rock intact around it to explore in the lab for the rest of the body. Then the rocks were wrapped in plaster for the trip home. Opening these kinds of plaster coverings in the lab is much like encountering a time capsule. Bits and pieces of our life on the Arctic tundra are there, as are the field notes and scribbles we make on the specimen. Even the smell of the tundra comes wafting out of these packages as we crack the plaster open. Fred in Philadelphia and Bob in Chicago were scratching on different boulders at the same general time. From one of these Arctic blocks, Bob had pulled out a particular small bone in a big fin of the Fish (we hadn’t named it Tiktaalik yet). What made this cube-shaped blob of bone different 52 from any other fin bone was a joint at the end that had spaces for four other bones. That is, the blob looked scarily like a wrist bone—but the fins in the block that Bob was preparing were too jumbled to tell for sure. The next piece of evidence came from Philadelphia a week later. Fred, a magician with his dental tools, uncovered a whole fin in his block. At the right place, just at the end of the forearm bones, the fin had that bone. And that bone attached to four more beyond. We were staring at the origin of a piece of our own bodies inside this 375-million-year-old fish. We had a fish with a wrist. The bones of the front fin of Tiktaalik— a fish with a wrist. Over the next months, we were able to see much of the rest of the appendage. It was part fin, part limb. Our fish had fin webbing, but inside was a primitive version of Owen’s one bone–two bones–lotsa blobs–digits arrangement. Just as Darwin’s theory predicted: at the right time, at the right place, we had found intermediates between two apparently 53 different kinds of animals. Finding the fin was only the beginning of the discovery. The real fun for Ted, Farish, and me came from understanding what the fin did and how it worked, and in guessing why a wrist joint arose in the first place. Solutions to these puzzles are found in the structure of the bones and joints themselves. When we took the fin of Tiktaalik apart, we found something truly remarkable: all the joint surfaces were extremely well preserved. Tiktaalik has a shoulder, elbow, and wrist composed of the same bones as an upper arm, forearm, and wrist in a human. When we study the structure of these joints to assess how one bone moves against another, we see that Tiktaalik was specialized for a rather extraordinary function: it was capable of doing push- ups. When we do push-ups, our hands lie flush against the ground, our elbows are bent, and we use our chest muscles to move up and down. Tiktaalik’s body was capable of all of this. The elbow was capable of bending like ours, and the wrist was able to bend to make the fish’s “palm” lie flat against the ground. As for chest muscles, Tiktaalik likely had them in abundance. When we look at the shoulder and the underside of the arm bone at the point where they would have connected, we find massive crests and scars where the large pectoral muscles would have attached. Tiktaalik was able to “drop and give us twenty.” 54  A full-scale model of Tiktaalik’s body (top) and a drawing of its fin (bottom). This is a fin in which the shoulder, elbow, and proto-wrist were capable of performing a type of push-up. Why would a fish ever want to do a push-up? It helps to consider the rest of the animal. With a flat head, eyes on top, and ribs, Tiktaalik was likely built to navigate the bottom and shallows of streams or ponds, and even to flop around on the mudflats along the banks. Fins capable of supporting the body would have been very helpful indeed for a fish that needed to maneuver in all these environments. This interpretation also fits with the geology of the site where we found the fossils of Tiktaalik. The structure of the rock layers and the pattern of the grains in 55 the rocks themselves have the characteristic signature of a deposit that was originally formed by a shallow stream surrounded by large seasonal mudflats. But why live in these environments at all? What possessed fish to get out of the water or live in the margins? Think of this: virtually every fish swimming in these 375- million-year-old streams was a predator of some kind. Some were up to sixteen feet long, almost twice the size of the largest Tiktaalik. The most common fish species we find alongside Tiktaalik is seven feet long and has a head as wide as a basketball. The teeth are barbs the size of railroad spikes. Would you want to swim in these ancient streams? It is no exaggeration to say that this was a fish-eat-fish world. The strategies to succeed in this setting were pretty obvious: get big, get armor, or get out of the water. It looks as if our distant ancestors avoided the fight. But this conflict avoidance meant something much deeper to us. We can trace many of the structures of our own limbs to the fins of these fish. Bend your wrist back and forth. Open and close your hand. When you do this, you are using joints that first appeared in the fins of fish like Tiktaalik. Earlier, these joints did not exist. Later, we find them in limbs. Proceed from Tiktaalik to amphibians all the way to mammals, and one thing becomes abundantly clear: the earliest creature to have the bones of our upper arm, our forearm, even our wrist and palm, also had scales and fin webbing. That creature was a fish. 56  What do we make of the one bone–two bones–lotsa blobs–digits plan that Owen attributed to a Creator? Some fish, for example the lungfish, have the one bone at the base. Other fish, for example Eusthenopteron, have the one bone– two bones arrangement. Then there are creatures like Tiktaalik, with one bone–two bones–lotsa blobs. There isn’t just a single fish inside of our limbs; there is a whole aquarium. Owen’s blueprint was assembled in fish. Tiktaalik might be able to do a push-up, but it could never throw a baseball, play the piano, or walk on two legs. It is a long way from Tiktaalik to humanity. The important, and often surprising, fact is that most of the major bones humans use to walk, throw, or grasp first appear in animals tens to hundreds of millions of years before. The first bits of our upper arm and leg are in 380-million-year-old fish like Eusthenopteron. Tiktaalik reveals the early stages in the evolution of our wrist, palm, and finger area. The first true fingers and toes are seen in 365-million-year-old amphibians like Acanthostega. Finally, the full complement of wrist and ankle bones found in a human hand or foot is seen in reptiles more than 250 million years old. The basic skeleton of our hands and feet emerged over hundreds of millions of years, first in fish and later in amphibians and reptiles. But what are the major changes that enable us to use our hands or walk on two legs? How do these shifts come about? Let’s look at two simple examples from limbs for some answers. 57  We humans, like many other mammals, can rotate our thumb relative to our elbow. This simple function is very important for the use of our hands in everyday life. Imagine trying to eat, write, or throw a ball without being able to rotate your hand relative to your elbow. We can do this because one forearm bone, the radius, rotates along a pivot point at the elbow joint. The structure of the joint at the elbow is wonderfully designed for this function. At the end of our upper-arm bone, the humerus, lies a ball. The tip of the radius, which attaches here, forms a beautiful little socket that fits on the ball. This ball-and-socket joint allows the rotation of our hand, called pronation and supination. Where do we see the beginnings of this ability? In creatures like Tiktaalik. In Tiktaalik, the end of the humerus forms an elongated bump onto which a cup-shaped joint on the radius fits. When Tiktaalik bent its elbow, the end of its radius would rotate, or pronate, relative to the elbow. Refinements of this ability are seen in amphibians and reptiles, where the end of the humerus becomes a true ball, much like our own. Looking now at the hind limb, we find a key feature that gives us the capacity to walk, one we share with other mammals. Unlike fish and amphibians, our knees and elbows face in opposite directions. This feature is critical: think of trying to walk with your kneecap facing backward. A very different situation exists in fish like Eusthenopteron, where the equivalents of the knee and elbow face largely in the same direction. We start development with little limbs 58 oriented much like those in Eusthenopteron, with elbows and knees facing in the same direction. As we grow in the womb, our knees and elbows rotate to give us the state of affairs we see in humans today. Our bipedal pattern of walking uses the movements of our hips, knees, ankles, and foot bones to propel us forward in an upright stance unlike the sprawled posture of creatures like Tiktaalik. One big difference is the position of our hips. Our legs do not project sideways like those of a crocodile, amphibian, or fish; rather, they project underneath our bodies. This change in posture came about by changes to the hip joint, pelvis, and upper leg: our pelvis became bowl shaped, our hip socket became deep, our femur gained its distinctive neck, the feature that enables it to project under the body rather than to the side. Do the facts of our ancient history mean that humans are not special or unique among living creatures? Of course not. In fact, knowing something about the deep origins of humanity only adds to the remarkable fact of our existence: all of our extraordinary capabilities arose from basic components that evolved in ancient fish and other creatures. From common parts came a very unique construction. We are not separate from the rest of the living world; we are part of it down to our bones and, as we will see shortly, even our genes. In retrospect, the moment when I first saw the wrist of a fish was as meaningful as the first time I unwrapped the fingers of the cadaver back in the human anatomy lab. Both 59 times I was uncovering a deep connection between my humanity and another being. 60  CHAPTER THREE HANDY GENES While my colleagues and I were digging up the first Tiktaalik in the Arctic in July 2004, Randy Dahn, a researcher in my laboratory, was sweating it out on the South Side of Chicago doing genetic experiments on the embryos of sharks and skates, cousins of stingrays. You’ve probably seen small black egg cases, known as mermaid’s purses, on the beach. Inside the purse once lay an egg with yolk, which developed into an embryonic skate or ray. Over the years, Randy has spent hundreds of hours experimenting with the embryos inside these egg cases, often working well past midnight. During the fateful summer of 2004, Randy was taking these cases and injecting a molecular version of vitamin A into the eggs. After that he would let the eggs develop for several months until they hatched. His experiments may seem to be a bizarre way to spend the better part of a year, let alone for a young scientist to launch a promising scientific career. Why sharks? Why a form of vitamin A? 61  To make sense of these experiments, we need to step back and look at what we hope they might explain. What we are really getting at in this chapter is the recipe, written in our DNA, that builds our bodies from a single egg. When sperm fertilizes an egg, that fertilized egg does not contain a tiny hand, for instance. The hand is built from the information contained in that single cell. This takes us to a very profound problem. It is one thing to compare the bones of our hands with the bones in fish fins. What happens if you compare the genetic recipe that builds our hands with the recipe that builds a fish’s fin? To find answers to this question, just like Randy, we will follow a trail of discovery that takes us from our hands to the fins of sharks and even to the wings of flies. As we’ve seen, when we discover creatures that reveal different and often simpler versions of our bodies inside their own, a wonderfully direct window opens into the distant past. But there is a big limitation to working with fossils. We cannot do experiments on long-dead animals. Experiments are great because we can actually manipulate something to see the results. For this reason, my laboratory is split directly in two: half is devoted to fossils, the other half to embryos and DNA. Life in my lab can be schizophrenic. The locked cabinet that holds Tiktaalik specimens is adjacent to the freezer containing our precious DNA samples. Experiments with DNA have enormous potential to reveal inner fish. What if you could do an experiment in 62 which you treated the embryo of a fish with various chemicals and actually changed its body, making part of its fin look like a hand? What if you could show that the genes that build a fish’s fin are virtually the same as those that build our hands? We begin with an apparent puzzle. Our body is made up of hundreds of different kinds of cells. This cellular diversity gives our tissues and organs their distinct shapes and functions. The cells that make our bones, nerves, guts, and so on look and behave entirely differently. Despite these differences, there is a deep similarity among every cell inside our bodies: all of them contain exactly the same DNA. If DNA contains the information to build our bodies, tissues, and organs, how is it that cells as different as those found in muscle, nerve, and bone contain the same DNA? The answer lies in understanding what pieces of DNA (the genes) are actually turned on in every cell. A skin cell is different from a neuron because different genes are active in each cell. When a gene is turned on, it makes a protein that can affect what the cell looks like and how it behaves. Therefore, to understand what makes a cell in the eye different from a cell in the bones of the hand, we need to know about the genetic switches that control the activity of genes in each cell and tissue. Here’s the important fact: these genetic switches help to assemble us. At conception, we start as a single cell that contains all the DNA needed to build our body. The plan for that entire body unfolds via the instructions contained in 63 this single microscopic cell. To go from this generalized egg cell to a complete human, with trillions of specialized cells organized in just the right way, whole batteries of genes need to be turned on and off at just the right stages of development. Like a concerto composed of individual notes played by many instruments, our bodies are a composition of individual genes turning on and off inside each cell during our development. Genes are stretches of DNA contained in every cell of our bodies. This information is a boon to those who work to understand bodies, because we can now compare the activity of different genes to assess what kinds of changes are involved in the origin of new organs. Take limbs, for example. When we compare the ensemble of genes active in the development of a fish fin to those active in the development of a human hand, we can catalogue the genetic differences between fins and limbs. This kind of comparison gives us some likely culprits—the genetic 64 switches that may have changed during the origin of limbs. We can then study what these genes are doing in the embryo and how they might have changed. We can even do experiments in which we manipulate the genes to see how bodies actually change in response to different conditions or stimuli. To see the genes that build our hands and feet, we need to take a page from a script for the TV show CSI: Crime Scene Investigation—start at the body and work our way in. We will begin by looking at the structure of our limbs, and zoom all the way down to the tissues, cells, and genes that make it. MAKING HANDS Our limbs exist in three dimensions: they have a top and a bottom, a pinky side and a thumb side, a base and a tip. The bones at the tips, in our fingers, are different from the bones at the shoulder. Likewise, our hands are different from one side to the other. Our pinkies are shaped differently from our thumbs. The Holy Grail of our developmental research is to understand what genes differentiate the various bones of our limb, and what controls development in these three dimensions. What DNA actually makes a pinky different from a thumb? What makes our fingers distinct from our arm bones? If we can understand the genes that control such patterns, we will be 65 privy to the recipe that builds us. All the genetic switches that make fingers, arm bones, and toes do their thing during the third to eighth week after conception. Limbs begin their development as tiny buds that extend from our embryonic bodies. The buds grow over two weeks, until the tip forms a little paddle. Inside this paddle are millions of cells which will ultimately give rise to the skeleton, nerves, and muscles that we’ll have for the rest of our lives. The development of a limb, in this case a chicken wing. All of the key stages in the development of a wing skeleton happen inside the egg. To study how this pattern emerges, we need to look at embryos and sometimes interfere with their development to assess what happens when things go wrong. Moreover, we need to look at mutants and at their internal structures and genes, often by making whole mutant populations 66 through careful breeding. Obviously, we cannot study humans in these ways. The challenge for the pioneers in this field was to find the animals that could be useful windows into our own development. The first experimental embryologists interested in limbs in the 1930s and 1940s faced several problems. They needed an organism in which the limbs were accessible for observation and experiment. The embryo had to be relatively large, so that they could perform surgical procedures on it. Importantly, the embryo had to grow in a protected place, in a container that sheltered it from jostling and other environmental disturbances. Also, and critically, the embryos had to be abundant and available year-round. The obvious solution to this scientific need is at your local grocery store: chicken eggs. In the 1950s and 1960s a number of biologists, including Edgar Zwilling and John Saunders, did extraordinarily creative experiments on chicken eggs to understand how the pattern of the skeleton forms. This was an era of slice and dice. Embryos were cut up and various tissues moved about to see what effect this had on development. The approach involved very careful microsurgery, manipulating patches of tissue no more than a millimeter thick. In that way, by moving tissues about in the developing limb, Saunders and Zwilling uncovered some of the key mechanisms that build limbs as different as bird wings, whale flippers, and human hands. They discovered that two little patches of tissue 67 essentially control the development of the pattern of bones inside limbs. A strip of tissue at the extreme end of the limb bud is essential for all limb development. Remove it, and development stops. Remove it early, and we are left with only an upper arm, or a piece of an arm. Remove it slightly later, and we end up with an upper arm and a forearm. Remove it even later, and the arm is almost complete, except that the digits are short and deformed. Another experiment, initially done by Mary Gasseling in John Saunders’s laboratory, led to a powerful new line of research. Take a little patch of tissue from what will become the pinky side of a limb bud, early in development, and transplant it on the opposite side, just under where the first finger will form. Let the chick develop and form a wing. The result surprised nearly everybody. The wing developed normally except that it also had a full duplicate set of digits. Even more remarkable was the pattern of the digits: the new fingers were mirror images of the normal set. Obviously, something inside that patch of tissue, some molecule or gene, was able to direct the development of the pattern of the fingers. This result spawned a blizzard of new experiments, and we learned that this effect can be mimicked by a variety of other means. For example, take a chicken embryo and dab a little vitamin A on its limb bud, or simply inject vitamin A into the egg, and let the embryo develop. If you supply the vitamin A at the right concentration and at the right stage, you’ll get the same mirror-image duplication that Gasseling, Saunders, and 68 Zwilling got from the grafting experiments. This patch of tissue was named the zone of polarizing activity (ZPA). Essentially, the ZPA is a patch of tissue that causes the pinky side to be different from the thumb side. Obviously chicks do not have a pinky and a thumb. The terminology we use is to number the digits, with our pinky corresponding to digit five of other animals and our thumb corresponding to digit one. Moving a little patch of tissue called the ZPA causes the fingers to be duplicated. The ZPA drew interest because it appeared, in some way, to control the formation of fingers and toes. But how? Some people believed that the cells in the ZPA made a molecule that then spread across the limb to instruct cells to make different fingers. The key proposal was that it was the concentration of this unnamed molecule that was the 69 important factor. In areas close to the ZPA, where there is a high concentration of this molecule, cells would respond by making a pinky. In the opposite side of the developing hand, farther from the ZPA so that the molecule was more diffused, the cells would respond by making a thumb. Cells in the middle would each respond according to the concentration of this molecule to make the second, third, and fourth fingers. This concentration-dependent idea could be tested. In 1979, Denis Summerbell placed an extremely small piece of foil between the ZPA patch and the rest of the limb. The idea was to use this barrier to prevent any kind of molecule from diffusing from the ZPA to the other side. Summerbell studied what happened to the cells on each side of the barrier. Cells on the ZPA side formed digits. Cells on the opposite side often did not form digits; if they did, the digits were badly malformed. The conclusion was obvious. Something was emanating from the ZPA that controlled how the digits formed and what they looked like. To identify that something, researchers needed to look at DNA. THE DNA RECIPE That project was left to a new generation of scientists. Not until the 1990s, when new molecular techniques became available, was the genetic control for the ZPA’s operation unraveled. 70  A major breakthrough happened in 1993, when Cliff Tabin’s laboratory at Harvard started hunting for the genes that control the ZPA. Their prey was the molecular mechanisms that gave the ZPA its ability to make our pinky different from our thumb. By the time his group started to work in the early 1990s, a number of experiments like the ones I’ve described had led us to believe that some sort of molecule caused the whole thing. This was a grand theory, but nobody knew what this molecule was. People would propose one molecule after another, only to find that none was up to the job. Finally, the Tabin lab came up with a novel notion, and one very relevant to the theme of this book. Look to flies for the answer. Genetic experiments in the 1980s had revealed the wonderful pattern of gene activity that sculpts the body of a fly from a single-celled egg. The body of a fruit fly is organized from front to back, with the head at the front and the wings at the back. Whole batteries of genes are turned on and off during fly development, and this pattern of gene activity serves to demarcate the different regions of the fly. Tabin didn’t know it at the time, but two other laboratories—those of Andy MacMahon and Phil Ingham— had already come up with the same general idea independently. What emerged was a remarkably successful collaboration among three different lab groups. One of the fly genes caught the attention of Tabin, McMahon, and Ingham. They noted that this gene made one end of a body segment look different from the other. Fly geneticists 71 named it hedgehog. Doesn’t the function of hedgehog in the fly body—to make one region different from another— sound like what the ZPA does in making the pinky different from the thumb? That parallel was not lost on the three labs. So off they went, looking for a hedgehog gene in creatures like chickens, mice, and fish. Because the lab groups knew the structure of the fly’s hedgehog gene, they had a search image to help them single out the gene in chickens. Each gene has a distinctive sequence; using a number of molecular tools, the researchers could scan the chicken’s DNA for the hedgehog sequence. After a lot of trial and error, they found a chicken hedgehog gene. Just as paleontologists get to name new species, geneticists get to name new genes. The fly geneticists who discovered hedgehog had named it that because the flies with a mutation in the gene had bristles that reminded them of a little hedgehog. Tabin, McMahon, and Ingham named the chicken version of the gene Sonic hedgehog, after the Sega Genesis video game. Now came the fun question: What does Sonic hedgehog actually do in the limb? The Tabin group attached a dye to a molecule that would stick to the gene, enabling them to visualize where the gene is active in the limb. To their great surprise, they found that only cells in a tiny patch of the limb had gene activity: the ZPA. So the next steps became obvious. The patterns of activity in the Sonic hedgehog gene should mimic those of 72 the ZPA tissue itself. Recall that when you treat the limb with retinoic acid, a form of vitamin A, you get a ZPA active on the opposite side. Guess what happens when you treat a limb with retinoic acid, then map where Sonic hedgehog is active? Sonic hedgehog becomes active on both sides— pinky and thumb—just as the ZPA does when it is treated with retinoic acid. Knowing the structure of the chicken Sonic hedgehog gave other researchers the tools to look for it in everything else that has fingers, from frogs to humans. Every limbed animal has the Sonic hedgehog gene. And in every single animal that we have studied, Sonic hedgehog is active in the ZPA tissue. If Sonic hedgehog hadn’t turned on properly during the eighth week of your own development, then you either would have extra fingers or your pinky and thumb would look alike. Occasionally, when things go wrong with Sonic hedgehog, the hand ends up looking like a broad paddle with as many as twelve fingers that all look alike. We now know that Sonic hedgehog is one of dozens of genes that act to sculpt our limbs from shoulder to fingertip by turning on and off at the right time. Remarkably, work in chickens, frogs, and mice was telling us the same thing. The DNA recipe to build upper arms, forearms, wrists, and digits is virtually identical in every creature that has limbs. How far back can we trace Sonic hedgehog and the other bits of DNA that build limbs? Is this stuff active in building the skeleton of fish fins? Or are hands genetically completely different from fish fins? We saw an inner fish in 73 the anatomy of our arms and hands. What about the DNA that builds it? Enter Randy Dahn with his mermaid’s purses. GIVING SHARKS A HAND Randy Dahn entered my laboratory with a simple but very elegant idea: treat skate embryos just the way Cliff Tabin treated chicken eggs. Randy’s goal was to perform all the experiments on skates that chicken biologists had performed on chicken eggs, from Saunders and Zwilling’s tissue surgeries all the way to Cliff Tabin’s gene experiments. Skates develop in an egg with a kind of shell and a yolk. Skates even have big embryos, just as chickens do. Because of these convenient facts, we could apply to skates many of the genetic and experimental tools people had developed to understand chickens. What could we learn by comparing the development of a shark fin to that of a chicken leg? Even more relevant, what could we learn about ourselves from all this? Chickens, as Saunders, Zwilling, and Tabin showed, are a surprisingly good proxy for our own limbs. Everything that was discovered by Saunders and Zwilling’s cutting and grafting experiments and by Tabin’s DNA work applies to our own limbs as well: we have a ZPA, we have Sonic hedgehog, and both have a great bearing on our well-being. As we saw, a malfunctioning ZPA or a mutation in Sonic 74 hedgehog can cause major malformations in human hands. Randy wanted to determine how different the apparatus is that builds our hands. How deep is our connection to the rest of life? Is the recipe that builds our hands new, or does it, too, have deep roots in other creatures? If so, how deep? Sharks and their relatives are the earliest creatures that have fins with a skeleton inside. Ideally, to answer Randy’s question, you would want to bring a 400-million-year-old shark fossil into the laboratory, grind it up, and look at its genetic structure. Then you’d try to manipulate its fossil embryos to learn whether Sonic hedgehog is active in the same general place as in our limbs today. This would be a wonderful experiment, but it is impossible. We cannot extract DNA from fossils so old, and, even if we could, we could never find embryos of those fossil animals on which to do experiments. Living sharks and their relatives are the next best thing. Nobody would ever confuse a shark fin with a human hand: you couldn’t ask for two more different kinds of appendages. Not only are sharks and humans very distantly related, but also the skeletal structures of their appendages look nothing alike. Nothing even remotely similar to Owen’s one bone–two bones–lotsa blobs–digits pattern is inside a shark’s fin. Instead, the bones inside are shaped like rods, long and short, thin and wide. We call them bones even though they are made of cartilage (sharks and skates are known as cartilaginous fish, because their skeletons never turn into hard bone). If you want to assess whether Sonic 75 hedgehog’s role in limbs is unique to limbed animals, why not choose a species utterly different in almost every way? In addition, why not choose the species that is the most primitive living fish with any kind of paired appendage, whether fin or limb? Sharks fit both bills perfectly. Our first problem was a simple one. We needed a reliable source for the embryos of sharks and skates. Sharks proved difficult to obtain with any degree of regularity, but skates, their close relatives, were another matter. So we started with sharks and used skates as our supply of sharks dwindled. We found a supplier who would ship us every month or two a batch of twenty or thirty egg cases with embryos inside. We became a virtual cargo cult as we waited each month for our shipment of precious egg cases. Work by Tabin’s group and others gave Randy important clues to begin his search. Since Tabin’s work in 1993, people had found Sonic hedgehog in a number of different species, everything from fish to humans. With the knowledge of the structure of the gene, Randy was able to search all the DNA of the skate and shark for Sonic hedgehog. In a very short time he found it: a shark Sonic hedgehog gene. The key questions to answer were Where is Sonic hedgehog active?, and, even more important, What is it doing? The egg cases were put to use as Randy visualized where and when Sonic hedgehog is active in the development of skates. He first studied whether Sonic hedgehog turns on at 76 the same time in skate fin development as it does in chicken limbs. Yes, it does. Then he studied whether it is turned on in the patch of tissue at the back end of the fin, the equivalent of our pinky. Yes again. Now he did his vitamin A experiment. This was the million-dollar moment. If you treat the limb of a chicken or mammal with this compound, you get a patch of tissue that has Sonic hedgehog activity on the opposite side, and this result is coupled with a duplication of the bones. Randy injected the egg, waited a day or so, and then checked whether, as in chickens, the vitamin A caused Sonic hedgehog to turn on in the opposite side of the limb. It did. Now came the long wait. We knew that Sonic hedgehog was behaving the same way in our hands and in skates’ and sharks’ fins. But what would the effect of all this be on the skeleton? We would have to wait two months for the answer. The embryos were developing inside an opaque egg case. All we could tell was whether the creature was alive; the inside of the fin was invisible to us. The end result was a stunning example of similarity among us, sharks, and skates: a mirror-image fin. The dorsal fins duplicated their structures in a wonderful front- to-back pattern, the same kind we saw with experiments in limbs. Limbs duplicate a limb structure. Shark fins duplicate a shark fin structure as do skates. Sonic hedgehog has a similar effect in even the most different kinds of appendage skeletons found on earth today. One effect of Sonic hedgehog, you may recall, is to make 77 the fingers distinct from one another. As we saw with respect to the ZPA, what kind of digit develops depends on how close the digit is to the source of Sonic hedgehog. A normal adult skate fin contains many skeletal rods, which all look alike. Could we make these rods different from one another, like our digits? Randy took a small bead impregnated with the protein made by Sonic hedgehog and put it in between these identical skeletal rods. The key to his experiment is that he used mouse Sonic hedgehog. So now we have a real contraption: a skate embryo with a bead inside that is gradually leaking mouse Sonic hedgehog protein. Would that mouse protein have any effect on a shark or a skate? There are two extreme outcomes to an experiment like this. One is that nothing happens. This would mean that skates are so different from mice that Sonic hedgehog protein has no effect. The other extreme outcome would present a stunning example of our inner fish. This outcome would be that the rods develop differently from one another, demonstrating that Sonic hedgehog does something similar in skates and in us. And let’s not forget that since Randy is using the protein from a mammal, it means that the genetic recipe would be really, really similar. Not only did the rods end up looking different from one another, they responded to Sonic hedgehog, much as fingers do, on the basis of how close they were to the Sonic hedgehog bead: the closer rods developed a different shape from the ones farther away. To top matters off, it was the 78 mouse protein that did the job so effectively in the skates. Normal fins (left) and Randy’s treated fins. The treated fins showed a mirror-image duplication just as chicken wings did. Photographs courtesy of Randall Dahn, University of Chicago. The “inner fish” that Randy found was not a single bone, or even a section of the skeleton. Randy’s inner fish lay in the biological tools that actually build fins. Experiment after experiment on creatures as different as mice, sharks, and flies shows us that the lessons of Sonic hedgehog are very general. All appendages, whether they are fins or limbs, are built by similar kinds of genes. What does this mean for the problem we looked at in the first two chapters—the 79 transition of fish fins into limbs? It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes. But there is a deeper beauty to these experiments on limbs and fins. Tabin’s lab used work in flies to find a gene in chickens that tells us about human birth defects. Randy used the Tabin lab discovery to tell us something about our connections to skates. An “inner fly” helped find an “inner chicken,” which ultimately helped Randy find an “inner skate.” The connections among living creatures run deep. 80  CHAPTER FOUR TEETH EVERYWHERE The tooth gets short shrift in anatomy class: we spend all of five minutes on it. In the pantheon of favorite organs—I’ll leave it to each of you to make your list—teeth rarely reach the top five. Yet the little tooth contains so much of our connection to the rest of life that it is virtually impossible to understand our bodies without knowing teeth. Teeth also have special significance for me, because it was in searching for them that I first learned how to find fossils and how to run a fossil expedition. The job of teeth is to make bigger creatures into smaller pieces. When attached to a moving jaw, teeth slice, dice, and macerate. Mouths are only so big, and teeth enable creatures to eat things that are bigger than their mouths. This is particularly true of creatures that do not have hands or claws that can shred or cut things before they get to the mouth. True, big fish tend to eat littler fish. But teeth can be the great equalizer: smaller fish can munch on bigger fish if they have good teeth. Smaller fish can use their teeth to scrape scales, feed on particles, or take out whole chunks of 81 flesh from bigger fish. We can learn a lot about an animal by looking at its teeth. The bumps, pits, and ridges on teeth often reflect the diet. Carnivores, such as cats, have blade-like molars to cut meat, while plant eaters have a mouth full of flatter teeth that can macerate leaves and nuts. The informational value of teeth was not lost on the anatomists of history. The French anatomist Georges Cuvier once famously boasted that he could reconstruct an animal’s entire skeleton from a single tooth. This is a little over the top, but the general point is valid; teeth are a powerful window into an animal’s lifestyle. Human mouths reveal that we are all-purpose eaters, for we have several kinds of teeth. Our front teeth, the incisors, are flat blades specialized for cutting. The rearmost teeth, the molars, are flatter, with a distinctive pattern that can macerate plant or animal tissue. The premolars, in between, are intermediate in function between incisors and molars. The most remarkable thing about our mouths is the precision with which we chew. Open and close your mouth: your teeth always come together in the same position, with upper and lower teeth fitting together precisely. Because the upper and lower cusps, basins, and ridges match closely, we are able to break up food with maximal efficiency. In fact, a mismatch between upper and lower teeth can shatter our teeth, and enrich our d